Evaluation of an Inconel-625 reactor and its wall effects on ethanol reforming in supercritical water

Abstract

Continuous ethanol reforming in supercritical water was investigated in an Inconel-625 reactor without catalyst addition. Experiments were carried out for different operating conditions: temperatures from 450 to 600 C, pressures from 225 to 300 bar, ethanol concentrations from 2.5 to 10% wt, and feed flow rates from 0.88 to 2.88 g/min. Higher temperatures increased the conversion and the hydrogen production, but above 575 C the experiments failed due to excessive tar formation, which plugged the reactor and/or the back pressure regulator. The pressure effect in the range considered here was insignificant. Ethanol dehydrogenation to acetaldehyde was the main reaction pathway, followed by acetaldehyde decomposition to CH4 and CO. The catalytic effects of the reactor's wall were studied by varying the reactor's surface-to-volume ratio (S/V) by inserting Inconel wires. The results confirmed the significant catalytic effects of Inconel 625, especially above 525 C, and also demonstrated that at 550 C and for an S/V ratio of 3.1 1/mm, conversion up to 96% was achieved without carbon deposition problems. Finally, a kinetic study for the ethanol dehydrogenation was carried out. This kinetic study revealed that ethanol dehydrogenation occurs mostly through wall-catalyzed reactions and that the homogeneous reaction is negligible in the reactor system considered here. Finally, an Arrhenius expression for the rate constant of the wall-catalyzed ethanol dehydrogenation reaction is proposed.